Because 35 years ago when I entered the IT field, there were minicomputers but they were mostly used to feed data to mainframes. There were no microcomputers except hand-built hobby machines. If you weren't working on mainframes, you pretty much weren't working in IT.

If the "38" was in operation when you started, there were many other computer vendors still in business. At that time (in addition to the wide variety of mini-computers) there were still several different flavors of "mainframe" including Burroughs, RCA, Honeywell, CDC, NCR, Univac, and some others.